1. Field of the Invention
The present invention relates to a US-OS characteristic control system for a vehicle such as an automobile or the like, and more particularly relates to an improved system for controlling the US-OS characteristic to enhance both the response during transitional turning and the controllability and stability during turning.
2. Prior Art
As is known in the art, in a vehicle such as an automobile or the like, it is generally preferable to set the US-OS characteristic to over-steer to enhance the response during transitional turning, while on the other hand, to set it to under-steer to enhance the controllability and stability during turning. In the Japanese Laying Open Publication No. Showa 62-198511, a suspension control system is described which is adapted to control the US-OS characteristic to be set to over-steer or neutral-steer in the initial stage of turning and to be set to under-steer in the final stage of turning. According to such a suspension control system, in the initial stage of turning the US-OS characteristic is set to over-steer or neutral-steer to enable a sharp turning and in the final stage of turning the characteristic is set to under-steer to assure the controllability and stability.
In the above described suspension control system, however, whether the turning state is in the initial stage or in the final stage is decided by determining whether or not the changing rate of turning state quantity such as differential value of yaw rate is larger and, accordingly, the US-OS characteristic is determined solely based upon the changing rate of turning state quantity. Therefore, it is difficult to enhance both the response during transitional turning and the controllability and stability during turning in a compatible manner. For example, in the afore mentioned suspension control system, since the US-OS characteristic is controlled to be over-steer or neutral-steer when the differential value of yaw rate exceeds a predeterminate value, if a steering angle increased or decreased during turning under the condition where the lateral acceleration of the vehicle is relatively large, the US-OS characteristic is set to over-steer or neutral-steer, which will deteriorate the controllability and stability of the vehicle.